This invention relates to a laser beam focusing apparatus and process for reading optical codes on objects carried on a supporting plane.
Throughout this description and the appended claims, the term "optical code" will be used to indicate a code (such as a bar code, two-dimensional code, color code, or the like) adapted to univocally identify objects bearing that code. In particular, reference will be made to bar codes for convenience of illustration.
Especially in the material handling and sorting plants, there is the problem of enabling the identification and classification of objects, which may differ considerably in height from one another, by means of a single instrument (typically a laser scanner), so as to speed up and optimize the subsequent object sorting and storage operations. In such situations, a fixed optics type of laser scanner not always allows codes to be brought into focus on the objects to be identified.
Further problems are encountered where a fixed optics scanner is used for reading high-resolution bar codes. In this situation, the achievable depth of field sharply decreases with decreasing of the code bars width. For example, with code bars 0.3 mm wide, a depth of field of no more than 300 mm can be achieved, while with code bars 0.2 mm wide, the maximum depth of field achievable is virtually reduced to half.
In practical applications an increased depth of field is required.
Various arrangements have been proposed heretofore for increasing the depth of field of a laser scanner. Best known among these are arrangements based on the multi-laser technique and on the electro-mechanical driving of optical parts effective to focus the laser beam.
Among the known multi-laser systems, that described in U.S. Pat. No. 5,483,051 of the same Applicant is of special interest. This system utilizes a modulated light type of laser scanner associated with two or more laser sources, each adapted to generate scans focused on preset distances. Such a system allows to measure the distance from the scanner of an optical code to be read, and to utilize this information for selecting and activating the one of the laser sources which is focused on that distance.
While obviating the prior art drawbacks, due in particular to the need of providing outside the scanner an array of photocells for measuring the height of the object bearing the bar code, or alternatively, to the need of providing for continuous and sequential activation of all the laser sources until a given scan recognizes the presence of a code, the multi-laser system of the above type has a drawback in that it involves a complicated optical construction and, hence, high costs. Both factors are restrictive of the applicability of such a system to small-size and of limited capacity plants.
The systems based on the electro-mechanical driving of optical parts for focusing the laser beam onto the object are indeed more simple construction-wise and economical to manufacture.
Although substantially achieving their objective, the last-mentioned systems also exhibit a series of drawbacks that have been left without a remedy.
A first drawback is associated with the need of an external height or distance sensor (depending on the mounting position of the scanner relative to the conveyor belt) adapted to drive the movements of the optical parts for focusing the laser beam. The presence of this sensor complicates the system and makes difficult, and some time impossible, the reading of irregularly shaped objects. In fact, the external sensor can only detect the maximum height of the object being conveyed under the reading area. Since the scanner is to focus the optical system based on this information, the error involved is surely high because a significant portion of the working depth of field cannot be utilized as expected, while with objects having an irregular surface shape, focusing will never be optimized.
A further drawback of conventional electro-mechanical driving systems is associated with the high response time of such systems, which makes them unsuitable for high speed applications.